The accumulation of carbon-based deposits has been a problem faced by many industries dealing with carbon-based products, either by using or producing them. Such carbon-based products may include, but not limited, to hydrocarbons.
In the petroleum industry, the buildup of coke causes significant problems, for instance, increasing pressure drop and scaling which compromise process efficiency and increases process downtime. To tackle such issues, significant cost is typically incurred for cleaning off the coke or replacing pipelines.
In another instance, severe carbon-based deposition or accumulation of coke may choke the tubes and nozzles. This may in turn lead to catastrophic consequences such as in the aviation industry. A safety alert for an operator report, published by the Federal Aviation Administration of the United States of America, described an incident where an Edelweiss A330 suffered from an uncontained engine failure during departure due to a rupture of the oil vent tube of the RB211 Trent 700 series turbofan engine. This was attributed to the blockage of an oil line caused by carbon-based deposits that led to significant loss of engine oil and consequently the uncontained engine failure.
One of the underlying causes for coke buildup may be due to the thermal degradation of hydrocarbon fluids. This problem may be commonly encountered, for example, when a hydrocarbon fluid comes into contact with a metal surface of a pipe and is exposed to heat over a long duration. This may be particularly prevalent in the automobile industry where intense residual heat radiated from an engine after shutting down (also known as heat soak) leads to fuel decomposition and coke deposition on injectors.
Similarly, coke deposition may be found in the turbines of an aircraft when the lubricant degrades in pipes due to the presence of a heat soak. The deposited coke particles may also produce an erosive effect when they get transported along a pipe in the hydrocarbon fluid, acting like abrasives and consequently wearing out other metallic parts of the aircraft system.
To develop a passivated surface for resisting coking on metal parts or tubing, a general understanding of the coking mechanism may be needed. Although the thermal decomposition of a hydrocarbon may involve complex elimination and radical chain reactions, these reactions tend to produce bulky cross-linked organic aggregates before being converted to coke particles.
In addition, the presence of metal oxides and carbon particles in the hydrocarbon fluid may accelerate the agglomeration of bulky organic aggregates as these bulky and heavy aggregates tend to possess strong adhesion and gelation tendencies. Thus, these aggregates are easily entrapped in micro-cracks and pits on a surface via physical anchoring and subsequent deposition via additional aromatization reactions may occur to cause more particles to be deposited.
Quite a handful of solutions have been developed to address the above problem and they may be classified into three types.
The first type may be categorized as a physical intervention method. An example of such a method, particularly an improved anti-coking air injection system, has been developed by General Motors. In this system, individual air flow is introduced into the engine cylinder head with electrical resistance heating elements that are periodically activated to develop radiant energy to burn off coke deposits that may form on passage side-walls.
The second category may be known as chemical inhibition which tends to rely on the addition of a particular additive into the fluid, such as a hydrocarbon stream, for inhibiting thermal degradation of the hydrocarbon that may lead to formation of coke deposits. Some examples of known additives are sulfiding agent, dihydroxybenzene, and phosphoric amide (I).
Despite the above, both categories of solution may be effective only in situations where there is a short heating duration. It may also be inappropriate for systems having static lubricants in a pipeline that is likely to be exposed to heat soak having temperatures that are capable of decomposing hydrocarbons.
In the third category, coating or reacting a surface to form a passivation barrier layer is employed.
In such methods, ceramic materials tend to be chosen since they are likely to be thermally stable and chemically unreactive. The selected ceramic material may be coated on the surface by known vapor deposition processes such as chemical or physical vapor deposition (CVD or PVD) or atomic layer deposition (ALD). Modified sol-gel coating approaches may work but variation in temperatures still undermines this approach.
Notwithstanding the above, this third strategy remains generally costly and may be difficult to apply when it comes to the interior surface of narrow pipes.
Other conventional methods may include flushing the pipe solely using an acid or a combination of acids. These acidic compositions may comprise at least one mineral acid or organic acids. However, these acidic compositions tend to be either too acidic such that the metal surface ends up being corroded or too mild such that the metal surface suffers from insufficient cleaning.
Accordingly, there is a need to overcome the limitations of existing solutions as proposed above. There is also need for enhancing a surface that becomes capable of mitigating or resisting carbon-based deposits and accumulation without suffering from these limitations.